exit trap in dash vs ksh and bash

zsh, pdksh (though not recent versions of mksh derived from that), yash, the Bourne shell behave like dash.

Only bash, ksh88, ksh93 and mksh behave otherwise.

The POSIX spec is not clear on what should be the correct behaviour, but there's nothing in there that says that the shell is allowed to override the default handler for the SIGINT (or other) signal.

It says the EXIT trap action should be evaluated when exit is invoked, but AFAICT, it doesn't even say for instance if it should be evaluated when the shell exits as the result of set -e or set -u or error conditions like syntax errors or failing special builtins.

To be able to run the EXIT trap upon reception of a signal, the shell would need to install a handler on that signal.

That's what ksh, mksh and bash do, but the list of signals they handle is different between all three implementations. The only signals common between all 3 seem to be INT, QUIT, TERM, ALRM and HUP.

If you want the EXIT trap to be run upon some signals, the portable way would be to handle those signals yourself:

trap 'cleanup' EXIT

That approach however doesn't work with zsh, which doesn't run the EXIT trap if exit is called from a trap handler.

It also fails to report your death-by-signal to your parent.

So instead, you could do:

for sig in INT QUIT HUP TERM ALRM USR1; do
  trap "
    trap - $sig EXIT
    kill -s $sig "'"$$"' "$sig"
trap cleanup EXIT

Now, beware though that if more signals arrive while you're executing cleanup, cleanup may be run again. You may want to make sure your cleanup works correctly if invoked several times and/or ignore signals during its execution.